Method of manufacturing a high lead count circuit board

ABSTRACT

A method for mounting an electronic component comprising providing an electrically insulating substrate having a recess for mounting the electronic component therein, a plurality of first leads disposed in the substrate side-by-side with a space therebetween and a plurality of second leads disposed between the first leads. Each first lead has an outer end projecting outwardly from the substrate and an inner end embedded within the substrate. Each second lead has an outer end outwardly extending from the substrate and an inner end exposed within the recess so that an electrical connection to the electronic component can be provided. One surface of the substrate has attached thereto a plurality of conductors, each having an outer end and an inner end disposed close to the recess for electrical connection to the electronic component. The inner ends of the first leads are connected to the outer ends of the conductors by through-hole conductors which penetrate the substrate.

This application is a divisional of application Ser. No. 07/646,355,filed Jan. 28, 1991, now U.S. Pat. No. 5,151,771.

BACKGROUND OF THE INVENTION

This invention relates to a circuit board for mounting an electroniccomponent and a method of manufacturing such a circuit board. Moreparticularly, this invention relates to a circuit board having a largenumber of leads for connecting one or more electronic componentsattached to the board to an external circuit.

FIGS. 1 and 2 illustrate one example of a conventional circuit board 110for mounting an electronic component such as a semiconductor chip. Asshown in the figures, a plurality of leads 111 have integrally formed ontheir inner ends inner connection portions 112 and a substrate 113 isprovided on both sides of the inner connection portions 112 so that eachlead 111 projects from the substrate 113. Also, the inner connectionportions 112 are electrically connected to conductors 114 formed on atleast one surface of the substrate 113 by through holes 115 so that theinner ends of the conductors 114 may be used as bonding terminals 117 towhich bonding wires B for electrically connecting an electroniccomponent A are bonded.

In recent years, it has become very desirable to increase the number ofouter leads (input and output pins) of the circuit board, so that a highspeed electronic component having a large number of electrodes can bemounted.

However, in the conventional electronic component mounting circuit board110 as described above, even though the width of the leads 111 is madenarrow and the distance between the leads 111 is made small in order toprovide as many outer leads 111 as possible on the substrate 113, eachlead 111 needs one through-hole connector 115 for internally connectingthe leads 111 and the conductors 114. Because the through-hole connector115 requires a space larger than that for the outer leads 111, thenumber of the outer leads 111 is still limited by the through-holeconnectors 115. A standard dimensional relationship of the transversedimension of the through-hole connector 115 and the distance between theleads 111 relative to the width of the lead 111 is 2:1.33:1.

FIG. 3 illustrates another known arrangement of the circuit board 110afor mounting an electronic component A, in which an additionalinsulating substrate 113a having conductors 114a is attached on the topsurface of the first substrate 113 and each conductor 114a on the uppersubstrate 113a is electrically connected by a longer through-holeconductor 115a to the corresponding outer lead 111 to provide a bondingterminal 117a to which the bonding wire B is bonded. Thus, the area inwhich the bonding wires B and the bonding terminals 117 and 117a aremost crowded is made in a "stadium" structure in which the bondingterminals 117 and 117a are located at the different levels for easy wirebonding as illustrated in FIG. 3. Even in this arrangement, however, onethrough-hole connector 115 or 115a is required for each of the outerleads 111 and the above-discussed problems remain unsolved. Moreover,since the conductors 114 and 114a as well as the bonding terminals 117and 117a are located at different levels with an electrically insulatingsubstrate therebetween, the overall thickness and the manufacturing costof the circuit board is increased, increasing the bulk of the device.

Also, Japanese Patent Laid-Open No. 59-98543 discloses a circuit boardfor mounting an electronic component in which a large number of leadsare provided by bonding thin conductive layers to the lead frame with aninsulating layer therebetween. However, since the outer leads have amulti-layered structure in which thin conductor layers are bonded to thelead frame with the insulating layer sandwiched therebetween, theinsulating layer may crack when it is bent at the time of supplying asparts, whereby the electrical insulation between the lead frame and theconductive layer is degraded. Accordingly, the inner and the outer leadsmust be wired in the same order (i.e., not in a crossing relationship)so as to prevent the inner and outer leads from contacting each other,thereby limiting freedom of design.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method formanufacturing a circuit board for mounting an electronic component freefrom the above-discussed problems.

The present invention resides in a method for manufacturing a circuitboard for mounting an electronic component which comprises the step ofproviding a lead frame having a plurality of leads including a pluralityof first leads disposed in a substantially side-by-side relationshipwith a space therebetween and a plurality of second leads disposedbetween the first leads with a space therebetween. Each of the firstleads has an outer end for external electrical connection and an innerend having a connection pad. Each of the second leads has an outer endfor external electrical connection and an inner end inwardly extendingfor electrical connection to the electronic component. The method alsocomprises the step of forming an electrically insulating substrate onthe lead frame so that it has first and second major surfacessubstantially parallel to the lead frame and the step of forming aplurality of first conductors, each attached to the first major surfaceof the substrate, and a plurality of through-hole conductors, eachextending through the substrate to electrically connect the inner end ofa first lead to the outer end of a first conductor. In order toaccommodate the electronic component therein, the method comprises thestep of forming a recess in the first major surface of the substrate,the recess also exposing the inner ends of the second leads forelectrical connection to the electronic component. In addition, themethod comprises the step of removing an outer region of the substrateto allow the outer ends of the first and second leads to project fromthe substrate for external electrical connection.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent from thefollowing detailed description of several embodiments of the presentinvention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional side view of a conventional circuit board formounting an electronic component taken along line I--I of FIG. 2;

FIG. 2 is a fragmental plan view of the conventional circuit board formounting an electronic component illustrated in FIG. 1;

FIG. 3 is a sectional side view of another conventional circuit boardfor mounting an electronic component;

FIG. 4 is a sectional side view of a circuit board for mounting anelectronic component of the present invention taken along line IV--IV ofFIG. 5;

FIG. 5 is a fragmental plan view of the circuit board illustrated inFIG. 4;

FIG. 6 is a fragmental plan view of a lead frame for use with thecircuit board of the present invention;

FIG. 7 is a sectional view of the lead frame illustrated in FIG. 6 takenalong line X--X of FIG. 6 and illustrating how the separation layer isformed;

FIG. 8 is a sectional view illustrating how the substrate and theconductor layer are formed on the lead frame illustrated in FIG. 7;

FIG. 9 is a sectional view illustrating how the recess is formed in thesubstrate and the inner ends of the outer leads are exposed;

FIG. 10 is a sectional view illustrating how the outer peripheralportion of the substrate shown in FIG. 9 is removed;

FIG. 11 is a sectional view of the lead frame illustrated in FIG. 6 andcorresponding to FIG. 7 but taken along line Y--Y of FIG. 6;

FIG. 12 is a sectional view illustrating how the substrate and theconductor layer together with the through holes are formed on the leadframe illustrated in FIG. 11;

FIG. 13 is a sectional view illustrating how the recess is formed in thesubstrate illustrated in FIG. 12;

FIG. 14 is a sectional view illustrating how the outer peripheralportion of the substrate illustrated in FIG. 13 is removed;

FIG. 15 is a sectional view of the lead frame with a centrally disposedseparation layer; and

FIG. 16 is a sectional view of the circuit board of another embodimentof the present invention.

DESCRIPTION OF EMBODIMENTS

FIGS. 4 and 5 illustrate a circuit board 10 of the present inventionwith an electronic component A such as a semiconductor chip mountedthereon. The circuit board 10 comprises an electrically insulatingsubstrate 13 which may have any convenient shape. In the illustratedembodiment, the substrate 13 is substantially square and has first andsecond major surfaces 13a and 13b which are generally parallel to andoppose one another. The substrate 13 also has outer peripheral sidesurfaces 13c and a recess 13d formed in the center of the first majorsurface 13a for mounting the electronic component A therein. The recessmay have any suitable shape for accommodating the electronic component,including, for example, a substantially square shape. The substrate 13may be made of a suitable well known epoxy resin material.

The circuit board 10 also comprises a plurality of leads which may beformed from any suitably conductive material, such as copper. The leadsinclude a plurality of first leads 11 disposed in the substrate 13 in asubstantially side-by-side relationship with a space therebetween and aplurality of second leads 12 disposed between the first leads 11 asviewed perpendicularly from the first major surface 13a of the substrate13. The plurality of first leads and the plurality of second leads arepreferably coplanar. Each of the first leads 11 has an outer lead 11aoutwardly extending from the outer peripheral side surface 13c to theexterior of the substrate 13 and an inner lead 11b embedded within thesubstrate 13. As best seen from FIG. 6, the inner lead 11b has asubstantially circular enlarged area 11c for an electrical connectionwhich will be described later. Each of the second leads 12 has an outerlead 12a outwardly extending from the outer peripheral side surface 13cto the exterior of the substrate 13 and an inner lead 12b extendingthrough the substrate 13 and exposed at its inner extremity within therecess 13d. The exposed ends of the inner leads 12b provide bonding pads12c for attaching bonding wires B for electrical connection to theelectronic component A.

The circuit board 10 further comprises a plurality of elongatedconductors 14 which are also formed from a suitably conductive material,such as copper. Each conductor 14 is attached to the first major surface13a of the substrate 13 and has an outer end 14a and an inner end 14b.The inner end 14b extends toward the electronic component A and isterminated in the vicinity of the recess 13d to provide a bonding pad14c to which a bonding wire B for electrically connecting the electroniccomponent A is bonded. The outer end 14a has a substantially circularenlarged area 14c at its extremity and the enlarged area 14d is disposedin registry with the enlarged area 11c of the first lead 11 within thesubstrate 13.

The elongated conductors 14 are electrically connected to the first lead11 by a plurality of through-hole conductors 15, which extend throughthe substrate 13 substantially perpendicularly to the plane of the majorsurface 13a. Each of the through-hole conductors 15 is electricallyconnected at one end 15a thereof to the enlarged area 14d of the outerend 14a of the conductor 14 and at its middle portion to the enlargedarea 11c of the inner end 11b of the first lead 11. The other end 15b ofthe through-hole conductor 15 extends through the substrate 13 and isexposed to the second major surface 13b of the substrate 13.

FIGS. 6 to 14 illustrate various steps of the method for manufacturingthe circuit board 10 illustrated in FIGS. 4 and 5 of the presentinvention.

FIG. 6 is a fragmental plan view of a lead frame 18 in which the firstleads 11 and the second leads 12 are integrally formed in apredetermined side-by-side pattern in a copper sheet. It is seen thatthe outer ends 11a of the first leads 11 and the outer ends 12a of thesecond leads 12 are connected to each other by connecting pieces 19,such as tie bars, to maintain each of the leads 11 and 12 in apredetermined position. Each of the inner ends 11b of the first leads 11has a substantially circular connection pad 11c having a diameter largerthan the width of the first lead 11, and each of the inner ends 12b ofthe second leads 12 inwardly extends so that it can be exposed in therecess 13d (FIGS. 4 and 5) and serve as the bonding pad 12c forelectrical connection to the electronic component A.

FIGS. 7 to 10 illustrate the circuit board of the present invention atvarious manufacturing steps substantially taken along line X--X of FIG.6, and FIGS. 11 to 14 illustrate the circuit board of the presentinvention at manufacturing steps corresponding to FIGS. 7 to 10 butsubstantially taken along line Y--Y of FIG. 6.

In FIGS. 7 and 11, a separator layer 20 is applied on both sides of thelead frame 18 at an outer peripheral region corresponding to the outerends 11a and 12a of the first and second leads 11 and 12. The separatorlayer 20 facilitates removal of a section of the substrate 13 to exposethe outer ends 11a and 12a of the leads 11 and 12 and may bepolyphenylenesulphide (PPS) for example.

In FIGS. 8 and 12, an electrically insulating substrate 13 having firstand second major surfaces 13a and 13b substantially parallel to the leadframe 18 is formed on the lead frame 18 so that the entire lead frame 18is centrally embedded within the substrate 13. The substrate 13 may beany well known resin material. Then, the through-hole conductors 15together with the film conductors 14 are formed on selected areas of themajor surfaces 13a and 13b of the substrate 13 as well as on the innercylindrical surfaces of the through holes extending through thesubstrate 13. In particular, each of the plurality of through-holeconductors 15 extends through the substrate 13 from its first majorsurface 13a to the second major surface 13b to electrically connect therounded end 11c at the inner end 11b of the first lead 11 to the roundedend 14d at the outer end 14a of the conductors 14. The conductors 14 andthe through-hole conductors 15 may be separately formed in the orderdiscussed above or opposite thereto and may be formed by any suitablemethod, such as selective copper deposition.

In FIGS. 9 and 13, a recess 13d for mounting the electronic component Atherein is formed in the substrate 13 along the first major surface 13aby a laser beam (not shown). The recess may have any convenientconfiguration. For example, the recess 13d is substantially square inplan and has a stepped configuration in section so that a shelf portionof the recess 13d exposes the inner end 12b of the second leads 12 toprovide the bonding pads 12c for electrical connection to the electroniccomponent A (FIG. 4).

Finally, as illustrated in FIGS. 10 and 14, the outer region of thesubstrate 13 and the separate layer 20 corresponding to or covering theouter ends 11a and 12a of the leads 11 and 12 are removed by a laserbeam (not shown) to define the outer peripheral side surfaces 13c of thesubstrate 13. The outer ends 11a and 12a of the first and the secondleads 11 and 12 project outwardly from the substrate side surfaces 13cfor electrical external connection.

FIG. 15 illustrates a step corresponding to that illustrated in FIGS. 7and 11, in which a second separator layer 21 is provided on the centralregion 19 of the lead frame 18 corresponding to the recess 13daccommodating the electronic component. Then, a substrate 13, similar tothat illustrated in FIGS. 8 and 12, is formed on the lead frame 18. Thesecond separator layer 21 is useful in the first substrate removing stepin that it allows for easy removal of this section of the substrate 13without damaging the bonding surfaces or the bonding pads 12c of thesecond leads 12. After the bonding pads 12c are exposed by the firstremoving step in which a first recess is formed, a deeper but smallersecond recess is formed to create the stepped recess 13d illustrated inFIG. 4, for example. Whereupon, the bonding pads 12c at the inner endsof the second leads 12 are separated from the central region 19. Othersteps may be identical to those previously described.

FIG. 16 illustrates still another modification in which a metallic layer22, which is similar to the connecting piece 19 illustrated in FIG. 6,is provided at the center of a lead frame. This metallic layer 22 staysin the bottom surface of the recess 23d of the substrate 23 and servesas a grounding electrode for grounding the electronic component Athrough an electric circuit which extends from the metallic layer 22 toa grounding lead 24 directly connected to the metallic layer 22. Themetallic layer 22 also serves as a heat conductor through which thegenerated heat can escape. Therefore, while the signal line leads 11 and12 (only one second lead 12 is shown in FIG. 16) are separated from themetallic layer 22, the metallic layer 22 remains electrically andmechanically connected to the grounding lead 24, which preferably has arelatively large cross-sectional area for providing good thermalconduction.

What is claimed is:
 1. A method for manufacturing a circuit board formounting an electronic component, the method comprising the stepsof:providing a lead frame having a plurality of leads including aplurality of first leads disposed in a substantially side-by-siderelationship with a space therebetween and a plurality of second leadsdisposed between the first leads with a space therebetween, the firstleads each having an outer end for external electrical connection and aninner end having a connection pad, and the second leads each having anouter end for external electrical connection and an inner end inwardlyextending for electrical connection to the electronic component; formingan electrically insulating substrate on the lead frame to have first andsecond major surfaces substantially parallel to the lead frame; forminga plurality of first conductors and a plurality of through-holeconductors on the substrate, the first conductors each being attached tothe first major surface of the substrate and having an outer end and aninner end and the through-hole conductors each extending through thesubstrate to be electrically connected between the inner end of one ofthe first leads and the outer end of one of the first conductors;forming a recess in the first major surface of the substrate formounting the electronic component therein, the recess exposing the innerend of the second leads for electrical connection to the electroniccomponent; and removing an outer region of the substrate to allow theouter ends of the first and second leads to project from said substratefor external electrical connection.
 2. A method for manufacturing acircuit board for mounting an electronic component as claimed in claim 1further comprising the step of applying a separator layer on said leadframe at an outer region corresponding to said outer ends of said firstand second leads before said formation of said substrate.
 3. A methodfor manufacturing a circuit board for mounting an electronic componentas claimed in claim 1, further comprising the step of applying aseparator layer on said lead frame at a central region corresponding tosaid recess for mounting said electronic component before said formationof said substrate.
 4. A method for manufacturing a circuit board formounting an electronic component as claimed in claim 1 wherein saidrecess is formed by laser machining.
 5. A method for manufacturing acircuit board for mounting electronic components, said method comprisingthe steps of:selecting a lead frame having a plurality of through holes;forming a plurality of first leads spaced from each other on the leadframe, each having an outer portion and an inner portion; forming aplurality of second leads, each extending between a pair of first leadsand each having an outer portion and an inner portion; enclosing thelead frame with an electrically insulating substrate having a pluralityof through-holes so that the plurality of first and second leads areembedded in the substrate; placing through-hole conductors in thethrough-holes of the substrate and electrically connected to the innerportion of the plurality of first leads; forming elongated conductors ona surface of the insulating substrate so that the elongated conductorsare in electrical connection with the through-hole conductors; forming astep-like recess in the surface of the insulating substrate on which theelongated conductors are formed such that the inner portions of theplurality of second leads are exposed on the step portion of the recess;removing a portion of the insulating substrate sufficient to expose theouter portions of the plurality of first and second leads to facilitateelectrical connection.
 6. A method for manufacturing a circuit board asclaimed in claim 5 further comprising the step of applying a separatorlayer on the lead frame at an outer region corresponding to the outerportions of said first and second leads before said step of forming thesubstrate.
 7. A method for manufacturing a circuit board as claimed inclaim 5, further comprising the step of applying a separator layer onthe lead frame at a central region corresponding to the recess formounting the electronic component before said step of forming saidsubstrate.
 8. A method for manufacturing a circuit board as claimed inclaim 5 wherein the recess is formed by laser machining.
 9. A method formanufacturing a circuit board for mounting electronic components, saidmethod comprising the steps of:selecting a lead frame having a centralportion and an outer portion containing a plurality of through holes;forming a plurality of first leads spaced from each other on the leadframe, each having an outer portion and an inner portion; forming aplurality of second leads, each extending between a pair of first leadsand each having an outer portion and an inner portion; connecting theinner portion of the plurality of second leads to the central portion ofthe lead frame; enclosing the lead frame with an electrically insulatingsubstrate having a plurality of through-holes so that the plurality offirst and second leads are embedded in the substrate; placingthrough-hole conductors in the through-holes of the substrate andelectrically connected to the inner portion of the plurality of firstleads; forming elongated conductors on a surface of the insulatingsubstrate so that the elongated conductors are in electrical connectionwith the through-hole conductors; forming a first recess in the surfaceof the insulating substrate on which the elongated conductors are formedsuch that the central portion of the lead frame and a part of the innerportions of the plurality of second leads are exposed in the firstrecess; removing a portion of the insulating substrate sufficient toexpose the outer portions of the plurality of first and second leads tofacilitate electrical connection.